Self-Commissioning Procedure For Inductance Estimation In An Electrical Machine

ABSTRACT

A method of estimating inductances and flux linkages of an electrical machine supplied with drive currents via current regulators. Drive current values are measured and fed back to the regulators for closed-loop control. The method includes providing one current regulator with an alternating current value either for a direct axis current reference i* d  or for a quadrature axis current reference i* q  of the machine current vector, while providing another current regulator with a predetermined direct current value for the remaining one of the two current references i* d  and i* q . After a predetermined time, a varying signal is superimposed onto an output signal generated by the current regulator in response to the AC value. Finally, a contribution signal, which corresponds to the contribution of the superimposed varying signal to the drive currents, is determined, and the machine inductances and flux linkages are estimated on the basis of the contribution signal.

FIELD OF THE INVENTION

The present invention generally relates to a method and system ofestimating inductances and flux linkages of an electrical machine.

BACKGROUND OF THE INVENTION

The interest in synchronous reluctance motors is increasing as it mightbecome a candidate for replacing a conventional induction motor. Inorder to fully exploit the capabilities of the synchronous reluctancemotor, a frequency converter is needed and a closed-loop control shouldbe performed. However, closed-loop controls (especially speed-sensorlesscontrols) have to be provided with appropriate parameters to avoidinstabilities and to work properly. Usually, the parameters required bythe control are obtained through a series of experimental tests.

Nowadays, these tests are performed automatically by the frequencyconverter with a minimal intervention by an external human operator.Different test signals on the machine and post-processing methods areexploited in order to estimate and complete the set of the parameters.These automatically performed tests are generally referred to as“self-commissioning” or “ID-run”.

One of the major benefits of ID-runs is the possibility to conductstandstill tests, during which the machine is at complete standstill anddifferent signals are injected. In this operating mode, maximum safetyis obtained, and the motor can be tested on-site with direct connectionto a mechanical load. This is particularly beneficial when theapplication is re-vamped and only the frequency converter is replaced,while leaving the existing motor. In this case, there is no need toremove the motor from the plant.

In the specific case of the synchronous reluctance motor, some issuesarise. The machine has a strongly non-linear relation between currentand flux linkages, with saturation effects and cross-magnetizationeffects more pronounced.

An example is shown in FIG. 1, which illustrates the current-to-fluxlinkage curves for a synchronous reluctance motor obtained from thefinite element method analysis.

In FIG. 1, λ_(d) and λ_(q) are the flux linkages in the d and q axes,respectively, and i_(d) and i_(q) are the corresponding currents, i.e. adirect axis current component and a quadrature axis current component,respectively, of a motor current vector. The derivatives of λ_(d) andλ_(q) with respect to i_(d) and i_(q), respectively, return the value ofthe inductances L_(d) and L_(q). Inductances L_(d) and L_(q) on d and qaxes, respectively, are dependent on both currents i_(d) and i_(q). Inpractice, L_(d) will to the greatest part depend on i_(d) but also to asmaller extent on i_(q). This is referred to as a cross-couplingcross-magnetization effect.

For a correct closed-loop (speed-sensorless) control of the machine,knowledge of the inductances in any operating point is beneficial. Theinductance is generally defined as the ratio of the flux linkage overthe current; depending on the adopted control strategy, apparentinductances (ratio between large-signal values) or differentialinductances (ratio between small-signal values) might be needed. In anycase, it is clear from FIG. 1 that the inductances vary as a function ofthe operating point.

The left-hand side of FIG. 1 shows the flux linkage on the d axis, whilethe right hand side of FIG. 1 shows the flux linkage on the q axis.Saturation is more visible on the d axis due to the presence of moreiron material in the magnetic path, while the q axis has a more “linear”profile due to more air material in the magnetic path.

From FIG. 1, it can be deducted that a truly effective ID-run should becapable of estimating the inductances in any operating point. Thedrawback is that for each operating point where both currents i_(d) andi_(q) are different from zero, an electromagnetic torque is produced andthe motor starts rotating (if the mechanical load allows it), accordingto the torque and mechanical equations:

$\tau = {\frac{3}{2}{p\left( {{\lambda_{d}i_{q}} - {\lambda_{q}i_{d}}} \right)}}$$\tau = {\tau_{L} + {J_{m}\frac{\omega_{m}}{t}} + {B_{m}\omega_{m}}}$

where p is the number of pole pairs in the machine, τ is the torque,τ_(L) is the load torque, J_(m) is the mechanical inertia, B_(m) is theviscous friction and ω_(m) is the mechanical speed.

Current self-commissioning procedures are capable of estimating, atstandstill, the inductances where either i_(d) or i_(q) is zero, thuswhen no torque is produced. For all other operating points, torque rampsare induced in the motor, and the inductances are estimated during thespeed transient. Such operating condition is not at standstill, andmight require the motor to be disconnected from the mechanical load.

SUMMARY OF THE INVENTION

A general object of the present invention is to solve or at leastmitigate the above-described problems in the art.

This object is attained in a first aspect of the present invention by amethod for estimating an inductance and/or flux linkage of an electricalmachine, which electrical machine is supplied with drive currents via afirst current regulator and a second current regulator, and actualvalues of drive currents are measured and fed back to the two currentregulators such that closed-loop control is provided. The methodcomprises the steps of providing the first current regulator with analternating current (AC) value either for a direct axis currentreference i*_(d) or for a quadrature axis current reference i*_(q) of amachine current vector, while providing the second current regulatorwith a predetermined direct current (DC) value for the remaining one ofthe two current references i*_(d) and i*_(q). Subsequently, after apredetermined time period has expired, a varying signal is superimposedin a control path where the AC value is provided. Finally, acontribution signal which corresponds to the contribution of thesuperimposed varying signal to the drive currents, is determined, andthe inductance and/or the flux linkage is estimated on the basis of thecontribution signal.

This object is further attained in a second aspect of the presentinvention by a system for estimating an inductance and/or flux linkageof an electrical machine, which system comprises a first currentregulator and a second current regulator via which the electricalmachine is supplied with drive currents. Said two current regulators arearranged to receive measured values of the drive currents such thatclosed-loop control is provided. The first current regulator is furtherarranged to be supplied with an AC value either for a direct axiscurrent reference i*_(d) or for a quadrature axis current referencei*_(q) of a machine current vector, while the second current regulatoris arranged to be provided with a predetermined DC value for theremaining one of the two current references i*_(d) and i*_(q). Thesystem further comprises a signal-injecting device arranged tosuperimpose, after a predetermined time period has expired, a varyingsignal in a control path where the AC value is provided, and acalculating device arranged to determine a contribution signal whichcorresponds to the contribution of the superimposed varying signal tothe drive currents. The calculating device is further arranged toestimate the inductance and/or the flux linkage on the basis of thecontribution signal.

Thus, the present invention advantageously provides a self-commissioningprocedure for frequency converters connected to electrical machines forestimation of the inductances and flux linkages in the electricalmachine with saturation and cross-magnetization effects taken intoaccount. With the present invention, the electrical machine is operatingin standstill or quasi-standstill condition while the inductance and theflux linkage are estimated.

When either of the current regulators is provided with an AC signal, andthe remaining one is provided with a predetermined DC signal, the motorproduces an oscillating torque. The frequency of the AC signal ispreferably high enough to prevent rotation of the motor (i.e. astandstill condition prevails), or at least the rotation is controlledsuch that the motor is close to being at standstill (i.e. aquasi-standstill condition prevails). At the same time, the frequencyshould be sufficiently low to allow the current regulators to follow theAC signal(s) provided to the regulators.

The AC signal has a different frequency from the signal superimposedafter the current regulator. The superimposed signal is typically asinusoidal signal of a higher frequency than the AC signal. The lowerfrequency is meant to create an alternating torque fast enough toprevent the motor from rotating, or retain the motor oscillations in thequasi-standstill condition. The second frequency is used forsmall-signal perturbation and for estimation of the inductances.

In an embodiment of the present invention, a Goertzel algorithm is used,implying that the frequencies of the AC signal and the superimposedsignal should have no common divisor in order to avoid detection ofspurious harmonics of the AC signal in the signal of interest.

In an embodiment of the method of the present invention, the steps ofthe method are performed with a plurality of different amplitude valuesof the AC value. Advantageously, a number of operation points arethereby obtained.

In a further embodiment of the method of the present invention, thesteps of the method are performed with a plurality of differentpredetermined DC values. Advantageously, a number of operation pointsare thereby obtained.

In still another embodiment of the present invention, the steps of saidmethod are performed with an AC value provided for both the direct axiscurrent reference i*_(d) and the quadrature axis current referencei*_(q). In order to be able to subsequently control the motor withrespect to cross-coupling, saturation and cross-magnetization effects,it is beneficial to have as much information at hand as possibleregarding the above described parameters. It may thus be advantageous tohave access to a great number of motor operating points with respect toboth the direct axis current component i_(d) and the quadrature axiscurrent component i_(q). Operation of the motor, performed automaticallyby a control program, will be more efficient and exact with knowledge ofdifferent operating points and in particular with different operatingpoints for both current vector components.

Further, it is to be understood that one of the current references couldbe set to zero while the other reference is varied over a range ofvalues, and vice versa. This is particularly useful in case the currentregulator is a PI regulator, which needs to be tuned with respect toproportional and integral gain.

In yet a further embodiment of the present invention, hysteresisregulators are used instead of PI regulators. In the case of usinghysteresis regulators as current regulators, the signal superimposed ina control path where the AC value is applied, is typically an AC.

In still a further embodiment of the present invention, a speedregulator is provided with a motor speed reference, wherein actualvalues of the motor speed are measured and fed back to the speedregulator such that closed-loop control is provided. Finally, the firstcurrent regulator is provided with the output of the speed regulator asan AC value.

Additional embodiments of the present invention, as well as furtherfeatures and advantages, will be disclosed in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention and advantages thereof will now bedescribed by way of non-limiting examples, with reference to theaccompanying drawings in which:

FIG. 1 shows flux linkages as a function of direct axis and quadratureaxis currents, in the left-hand side graph the flux linkage on the daxis is shown and in the right-hand side graph the flux linkage on the qaxis is shown,

FIG. 2 shows a system for estimating inductances and flux linkages of anelectrical machine according to an embodiment of the present invention,

FIG. 3 illustrates a signal supplied to the electrical machine accordingto an embodiment of the present invention,

FIG. 4 shows a system for estimating inductances and flux linkages of anelectrical machine according to a further embodiment of the presentinvention, and

FIG. 5 shows a system for estimating inductances and flux linkages of anelectrical machine according to yet a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a control system 100 according to an embodiment of thepresent invention for controlling an electrical machine 110 such as asynchronous reluctance motor.

In this embodiment, a direct axis current reference i*_(d) of a motorcurrent vector is provided to a PI regulator 101, while a quadratureaxis current reference i*_(q) of the motor current vector is provided toa PI regulator 102. The direct axis current component path of thecontrol system 100 is responsible for controlling the flux linkage inthe machine, while the quadrature axis component path is responsible forcontrolling the machine torque.

In case tuning of the PI regulators is necessary, the tuning may beperformed by providing the PI regulator 101 with different predeterminedDC values for the direct axis current reference i*_(d) while providingthe PI regulator 102 with a zero value for the quadrature axis currentreference i*_(q). Hence, no torque is produced. The PI regulator 101 maythen be tuned through analysis of transient response of the direct axiscurrent component. This procedure is subsequently repeated by providingthe PI regulator 102 with different predetermined DC values for thequadrature axis current reference i*_(q) while providing the PIregulator 101 with a zero value for the direct axis current referencei*_(d) for tuning the PI regulator 102.

Further, before procuring motor operating points with both i*_(d)≠0 andi*_(q)≠0, it may be desired to estimate inductances and flux linkageswith one of the two current references at zero level while altering thelevel of the other current reference, and vice versa. Thus, differentpredetermined DC values for the direct axis current reference i*_(d) areprovided to the PI regulator 101, while the PI regulator 102 is providedwith a zero value for the quadrature axis current reference i*_(q).After the PI regulators, a small sinusoidal voltage signal u*_(d,osc) issuperimposed by a signal-injecting unit (not shown) onto to the voltageu*_(d) generated by the PI regulator 101. That is, the sinusoidalvoltage signal is superimposed in a control path where the predeterminedDC value is provided. A contribution signal i_(d,osc), which correspondsto the contribution of the superimposed sinusoidal voltage signal to thedrive currents, is determined from measured drive current signals at acalculation block 107 after appropriate transformations have beenundertaken. In particular, the contribution signal is determined byusing a so-called Goertzel algorithm, which effectively is a single-toneversion of the discrete Fourier transform. Thereafter, differentialinductance can be determined at the calculation block 107 by performing|u*_(d,osc)|/(ω_(osc)|i_(d,osc)|), where ω_(osc) is the angularfrequency of the superimposed voltage. Curve λ_(d)=f(i_(d), i_(q)) fori_(q)=0 may then be elaborated for different operating points. That is,due to the closed loop control which is utilized, the actual direct axiscurrent component i_(d) will ideally follow the predetermined DC valueprovided to the PI regulator 101. Respectively, the actual quadratureaxis current component i_(q) in this particular measurement will ideallyfollow the zero value provided to the PI regulator 102. The smallvoltage signal u*_(d,osc) with frequency ω_(osc) superimposed at theoutput of the PI regulator 101 will lead to generation of a smallcurrent signal i_(d,osc) with frequency ω_(osc) on top of the directaxis current component i_(d). Typically, the frequency ω_(osc) of thesuperimposed signal is selected such that the PI regulators will notcancel that small-scale sinusoidal signal.

Thereafter, the procedure is repeated by providing the PI regulator 101with a zero value for the direct axis current reference i*_(d), whichfacilitates derivation of λ_(q)=f(i_(d), i_(q)) for i_(d)=0 fordifferent operating points.

Before proceeding to describe estimation of inductances in otheroperating points, the remaining functional blocks of the control system100 of FIG. 2 will be described. Most of the functional blocks deal withvarious types of vector transformations. These transformations areperformed since they greatly simplify complexity of the mathematicalmodel of the system. Firstly, block 103 performs an inverse Parktransformation, which is a transformation from a rotating (d, q, θ) to astationary (α, β) reference frame, where θ is the rotor angle.

The inverse Park transformation block 103 is followed by a space vectormodulation (SVM) block 104. The space vector modulation (SVM) candirectly transform the stator voltage vectors from the two-phaseα,β-coordinate system into pulse-width modulation (PWM) signals. SVMgenerally involves inverse Clarke transformation.

Thereafter, the motor 110 is supplied with a set of PWM drive signals.Tracing back through the control path to the PI regulators, it can beseen that this set of drive signals is derived, via the transformationsdescribed in the above, from the sum signal u*_(d)+u*_(d,osc) and theregulated quadrature axis voltage component from the PI regulator 102,i.e. u*_(q).

The three motor currents are measured and fed back to the PI regulatorsvia Clarke transformation block 105 and Park transformation block 106.In practice, the instantaneous sum of the three current values is zero.Thus, with knowledge of two of the currents, the third can bedetermined. As is indicated in FIG. 2, current i_(c) is optional but inpractice typically omitted, since the cost of a third current sensor canbe avoided. The Clarke transform transforms a three axis (i_(a), i_(b),i_(c)), two dimensional coordinate system referenced to the motor statoronto a two axis (i_(α), i_(β)) system while maintaining the samereference.

This is followed by a Park transformation block 106, which transformsthe stationary reference frame (α, β) into the rotating reference frame(d, q, θ).

Finally, the actual values i_(d), i_(q) of the motor current vector arefed back to the respective PI regulators 101, 102, whereby closed-loopcontrol is accomplished.

When estimating inductances and flux linkages at various operatingpoints where both i_(d)≠0 and i_(q)≠0, the following procedure may beundertaken in accordance with an embodiment of the present invention.First, an alternating square-wave current value for the direct axiscurrent reference i*_(d) is provided to the PI regulator 101, while apredetermined DC value for the quadrature axis current reference i*_(q)is provided to the PI regulator 102. An “alternating current (AC) value”refers to a current signal constantly changing sign in contrast to a“predetermined direct current (DC) value” which refers to a currentsignal which does not change sign and is preferably constant. After thePI regulators, a small sinusoidal voltage signal u*_(d,osc) issuperimposed onto the voltage u*_(d) generated by the PI regulator 101after a predetermined time period has expired.

With reference to FIG. 3, this time period is in an embodiment 0.1 s,but the selection of time period depends on the specific situation towhich the present invention is applied. As can be seen in FIG. 3 whichillustrates the actual drive current of the motor (in this case thedirect axis current component i_(d) of the motor current vector), thecurrent is a “clean” square-wave for 0.1 s, tracing the direct axiscurrent reference i*_(d) provided to direct axis path of the controlsystem. After 0.1 s has expired, the contribution signal i_(d,osc) canbe seen in the form of a small-scale sinusoidal signal superimposed ontothe square-wave current component id.

The contribution signal i_(d,osc) is determined from the measured drivecurrent signals at a calculation block 107 after appropriatetransformations have been undertaken. Again, the contribution signal isdetermined using the Goertzel algorithm. Thereafter, differentialinductance can be determined at the calculation block 107 by performing|u*_(d,osc)|/(ω_(osc)|i_(d,osc)|) where ω_(osc) is the angular frequencyof the superimposed voltage. Again, due to the closed-loop control whichis utilized, the actual direct axis current component i_(d) will ideallyfollow the AC value provided to the PI regulator 101, which AC value hasbeen set to be a square-wave. The actual quadrature axis currentcomponent i_(q) will ideally assume the predetermined DC value providedto the PI regulator 102.

Advantageously, the procedure is repeated with different amplitudes ofthe square-wave, such that a great number of motor operating points areattained. The curve λ_(d)=f(i_(d), i_(q)) may then be elaborated withi_(q) corresponding to the predetermined DC value provided to the PIregulator 102. Even further advantageous is to repeat the procedure witha number of different predetermined DC values. The curve λ_(d)=f(i_(d),i_(q)) may then be elaborated for a number of different operatingpoints.

Thereafter, the procedure is repeated by providing the PI regulator 101with a predetermined DC value for the direct axis current referencei*_(d), and by providing the PI regulator 102 with an alternatingsquare-wave current value for the quadrature axis current referencei*_(q). Again, the procedure is advantageously repeated with a number ofdifferent square-wave amplitudes and a number of different predeterminedDC values.

FIG. 4 shows a further embodiment of the present invention, where thecontrol system 100 employs hysteresis regulators 109, 111, 112 insteadof the PI regulators used in the system illustrated with reference toFIG. 2. If an input signal to the hysteresis regulator is below a firstthreshold A, the output is zero, while if an input signal exceeds asecond threshold B (B>A), the output is 1. It should be noted that otheroutputs than 0 and 1 are possible in more elaborate hysteresisregulators. One of the differences of hysteresis regulators as comparedto PI regulators is that response time of hysteresis regulatorsgenerally is considerably shorter.

In the case of using hysteresis regulators, there is no need to gothrough the tuning procedure utilized for the PI regulators, which wasdescribed in connection to FIG. 2.

Again, an alternating square-wave current value for the direct axiscurrent reference i*_(d) is provided, in this case to block 103 whichperforms an inverse Park transformation. A predetermined DC value forthe quadrature axis current reference i*_(q) is likewise provided toblock 103. As in the case of the embodiment described with reference toFIG. 2, a varying signal i*_(d,osc) is superimposed by asignal-injecting unit (not shown) to the control path where the AC valueis provided, after a predetermined time period has expired. In thisparticular embodiment, the varying signal is provided to an input of theinverse Park transformation block 103. Thereafter, the currents aresupplied to the hysteresis regulators 109, 111, 112 via inverse Clarketransformation block 108. Before the motor 110 is supplied with a set ofdrive signals, a voltage inversion is performed at block 113.

The three motor currents are measured and supplied to Clarketransformation block 105 and Park transformation block 106. In practice,the instantaneous sum of the three current values is zero. Thus, withknowledge of two of the currents, the third can be determined. As isindicated in FIG. 4, current i_(c) is optional but in practice typicallyomitted, since the cost of a third current sensor can be avoided. TheClarke transform transforms a three axis (i_(a), i_(b), i_(c)), twodimensional coordinate system referenced to the motor stator onto a twoaxis (i_(α), i_(β)) system while maintaining the same reference.

This is followed by a Park transformation block 106, which transformsthe stationary reference frame (α, β) into the rotating reference frame(d, q, θ).

In this particular embodiment where hysteresis regulators 109, 111, 112are utilized, the actual values i_(a), i_(b) of the motor current arefed back to the respective hysteresis regulator, whereby closed-loopcontrol is accomplished.

The contribution signal i_(d,osc) is determined from the measured drivecurrent signals at a calculation block 107. Again, the contributionsignal is determined using the Goertzel algorithm. Thereafter,differential inductance can be determined at the calculation block 107by performing |u*_(d,osc)|/(ω_(osc)|i_(d,osc)|) where ω_(osc) is theangular frequency of the superimposed voltage. In order to attain thevoltage u*_(d,osc) required to determine the differential inductance,voltages u*_(a), u*_(b), u*_(c) output from the hysteresis regulatorsare measured and supplied to the calculation block 107 via Clarketransformation block 114 and Park transformation block 115. The Goertzelalgorithm is again applied to the resulting voltage signal in order toattain the voltage u*_(d,osc).

Advantageously, the procedure is repeated with different amplitudes ofthe square-wave, such that a great number of motor operating points areattained. The curve λ_(d)=f(i_(d), i_(q)) may then be elaborated withi_(q) corresponding to the predetermined DC value provided to the Parktransformation block 103 for the quadrature axis current referencei*_(q). Even further advantageous is to repeat the procedure with anumber of different predetermined DC values. The curve λ_(d)=f(i_(d),i_(q)) may then be elaborated for a number of different operatingpoints. Thereafter, the procedure is repeated by providing the Parktransformation block 103 with a predetermined DC value for the directaxis current reference i*_(d), and by providing the same Parktransformation block 103 with an alternating square-wave current valuefor the quadrature axis current reference i*_(q). Again, the procedureis advantageously repeated with a number of different square-waveamplitudes and a number of different predetermined DC values.

FIG. 5 illustrates a further embodiment of the present invention, whichshows a system 100 identical to that of FIG. 2 with the exception of afurther PI regulator 116 used for controlling speed of the motor 110. Amotor speed reference ω*_(m) is provided to the PI regulator 116, whilethe actual speed ω_(m) of the motor is measured and fed back to the PIregulator 116. The output of the PI regulator 116 serves as an AC valuefor any one of the current references, in this particular exemplifyingembodiment to direct axis current reference i*_(d). Thus, the PIregulator 116 can be used to modify the AC value that follows asquare-wave pattern. If the PI regulator 116 is configured to bedynamically slow, it can provide a slowly varying AC value that canmaintain the mean value of the motor speed at zero, or close to zero.The PI regulator 116 can further be implemented in the system utilizinghysteresis controllers according to FIG. 4.

It is to be understood that the method of the present inventiontypically is performed by means of a device comprising a processing unitarranged to perform the steps of the invention when appropriate programcode is downloaded to the processing unit. The processing unit may beembodied in the form of a general or special purpose computer, an ASIC,an FPGA, etc. Further, the functionality of the system of the presentinvention may be implemented by means of one or more such processingunits.

The skilled person in the art realizes that the present invention by nomeans is limited to the examples described hereinabove. On the contrary,many modifications and variations are possible within the scope of theappended claims.

1. A method for estimating an inductance and/or flux linkage of anelectrical machine, wherein the electrical machine is supplied withdrive currents via a first current regulator and a second currentregulator, and actual values of drive currents are measured and fed backto the two current regulators such that closed-loop control is provided,which method comprises the steps of: providing the first currentregulator with an AC value either for a direct axis current referencei*_(d) or for a quadrature axis current reference i*_(q) of a machinecurrent vector, while providing the second current regulator with apredetermined DC value for the remaining one of the two currentreferences i_(d) and i*_(q); superimposing, after a predetermined timeperiod has expired, a varying signal in a control path where the ACvalue is provided; and determining a contribution signal whichcorresponds to the contribution of the superimposed varying signal tothe drive currents, estimating the inductance and/or the flux linkage onthe basis of the contribution signal.
 2. The method of claim 1, whereinthe steps of said method are performed with a plurality of differentamplitude values of the AC value.
 3. The method of claim 1, wherein thesteps of said method are performed with a plurality of differentpredetermined DC values.
 4. The method of claim 1, wherein the steps ofsaid method are performed with an AC value provided for both the directaxis current reference and the quadrature axis current reference.
 5. Themethod of claim 1, wherein the AC value provided to the first currentregulator is a square-wave current.
 6. The method of claim 1, whereinthe two current regulators are PI regulators and the superimposedvarying signal is an alternating voltage superimposed on an outputsignal generated by the first current regulator in response to the ACvalue.
 7. The method of claim 1, wherein the current regulators arehysteresis regulators and the superimposed varying signal is analternating current.
 8. The method of claim 1, further comprising thesteps of: providing a speed regulator with a motor speed referenceω*_(m); measuring actual values of the motor speed ω_(m), and feedingback the measured actual speed values to the speed regulator such thatclosed-loop control is provided; and supplying the first currentregulator with the output of the speed regulator as an AC value.
 9. Themethod of claim 1, wherein the electrical machine is a synchronousreluctance motor.
 10. The method of claim 1, wherein the contributionsignal is determined using the Goertzel algorithm.
 11. A system forestimating an inductance and/or flux linkage of an electrical machine,which system comprises: a first current regulator and a second currentregulator via which the electrical machine is supplied with drivecurrents, said two current regulators being arranged to receive measuredvalues of the drive currents such that closed-loop control is provided,wherein the first current regulator is further arranged to be suppliedwith an AC value either for a direct axis current reference i*_(d) orfor a quadrature axis current reference i*_(q) of a machine currentvector, while the second current regulator is arranged to be providedwith a predetermined DC value for the remaining one of the two currentreferences i*_(d) and i*_(q); a signal-injecting device arranged tosuperimpose, after a predetermined time period has expired, a varyingsignal in a control path where the AC value is provided; and acalculating device arranged to determine a contribution signal whichcorresponds to the contribution of the superimposed varying signal tothe drive currents, and further arranged to estimate the inductanceand/or the flux linkage on the basis of the contribution signal.
 12. Thesystem of claim 11, wherein a square-wave AC value is provided to thefirst current regulator.
 13. The system of claim 11, wherein the twocurrent regulators are PI regulators and the superimposed varying signalis an alternating voltage superimposed on an output signal generated bythe first current regulator in response to the AC value.
 14. The systemof claim 11, wherein the current regulators are hysteresis regulatorsand the superimposed varying signal is an alternating current.
 15. Thesystem of claim 11, further comprising: a speed regulator arranged to beprovided with a motor speed reference ω*_(m) and further being arrangedto be supplied with actual values of the motor speed ω_(m) such thatclosed-loop control is provided, the first current regulator beingarranged to be supplied with the output of the speed regulator as an ACvalue.